SPring-8 レーザー電子光 ビームラインでの タギング検出器の性能評価 核物理研究センター 三部 勉 LEPS collaboration 日本物理学会 近畿大学 １．レーザー電子光 ２．タギング検出器 ３．実験セットアップ ４．エネルギー分解能 ５．検出効率とバックグラウンドレート ６．まとめ

Laser 3.5eV Scattered photon (Laser Electron Photon) Energy Max 2.4GeV Intensity 1×10 7 /sec Linear/Circular polarization Backward Compton scattering Electron 8GeV Laser Electron Photon

Position of Scattered electron （ SSDch ） Photon Energy （ GeV ） Photon energy E  = E e - E e’ Initial electron energy Scattered electron energy Analyzed by Bending Magnet Photon Tagging Measured by PWO calorimeter

Tagging counter (top view) Silicon Strip Detector ×2 (Analog readout,100  m pitch 640 ch) 1cm 8GeV electron beam Scattered electron Plastic scintillator ×20 (3mm(T),7.4mm(W),10 mm(H)) Energy Counter Timing Counter Accelerator chamber wall

LEPS spectrometer Analyzing magnet (0.7T) Vertex Detector (SSD) Charged particle tracking –MWDC (3 chambers) Particle Identification –TOF wall (40 slats)

Experimental Setup (Top view) TOF Dipole Magnet （０．７ T) MWDC Bremsstrahlung Photon Target （ Al ２ mm width, １ mm thick ） ＋ Scintillation fiber （ 1mm  ２） EE Tagging Counter Spectrometer （ e + e - ） e-e- e+e+ Energy Resolution Tagging efficiency Background rate Plastic Scintillator

e + e - Momentum Reconstruction E(Tagging) GeV e + e - production point → localized in wire target  x,  z ～ 0.3mm One to one correspondence p e x e Monte Carlo Simulation  p e ～ 8 E （ e + e - ） GeV P e : e+/e- momentum x e : e+/e- x-position after magnet

Energy Resolution   2 =  Meas 2 ｰ  e+e- 2 Energy resolution of Tagging Resolution of momentum reconstruction Width of Experimental Data Monte Carlo 50 ‐ 50 （ MeV ） 0 E(Tagging) - E(e+e-) Counts 1.60<E  <1.88 GeV  Meas =23MeV Ave.15.3 MeV   (MeV) E  (GeV) Systematic error

Electron Track in Tagging counter Scattered Electron SSD Plastic Scintillator Plastic Scintillator Event Selection / Analysis 1. Pick up two clusters in up and down stream SSD. 2. Angle of track |  e -  e | < 4  3. Number of corresponding hit in the scintillator array (n) ee → Hit pattern ‘quality’ = 0 SSD+ Scintillator(n = 0) =1 SSD+ Scintillator(n = 1) =2 SSD+ Scintillator(n = 2)

Tagging Efficiency and Background rate Background rate Hit pattern(quality) SSD+ Scintillator(n = 2) SSD+ Scintillator(n ≧ 1) SSD+ Scintillator(n ≧ 0) 86.5± 0.6% Efficiency 90.6± 0.5% 91.0± 0.5% 2.3 ± 0.1% 2.6 ± 0.1% 2.8 ± 0.1% Efficiency ＝ Number of tagger hit( ≧ 1 track) ÷ Number of e+e- pair in Tagging acceptance Background rate ＝ Number of tagger hit( ≧ 2 tracks) ÷ Number of tagger hit( ≧ 1 track) Definitions Condition : Bremsstrahlung photon ＠ maximum electron current

Summary Performance of Tagging system 1. Experimental method e+e- momentum reconstruction using wire target. 2. Energy resolution Photon Tagging Resolution 15 MeV 3. Efficiency and background rate (Bremsstrahlung photon) Efficiency 90% [SSD+ Scintillator (n ≧ 1)] Background rate 3 %. 4. Further study is on going with Laser Electron Photon...

Photon Energy Spectrum 1st beam observation (1999.July) Photon energy (GeV) Counts

Tagging counter PMT SSD Beam Plastic Scintillator